This application claims the priority of German patent application no. 10 2007 049 436.1, filed Oct. 16, 2007, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fiber laser arrangement having a high beaming power.
Fiber lasers are part of the class of so-called all-solid-state lasers, i.e., diode-pumped solid-state lasers. By constructing the laser with an active medium in a waveguide which acts as a fiber, and which consists of a doped core and a cladding for the guiding of the light, the thermal problems of conventional solid-state lasers for generating high power can be overcome. Because of their considerable length, such glass lasers with optical waveguide characteristics permit a very high amplification. Modern cladding-type pumping techniques with efficient laser diodes, effective cooling concepts and the coupling of several continuously operating individual fiber lasers, have made it possible to work with such fiber lasers in the short-wave range. Compare “Laser 2005” Fair, IPG-Photonics Corp.
Adaptive mirrors have been used to compensate for atmospheric turbulences in high-energy laser arrangements. Such adaptive mirrors are inserted into the beam path on the output side of the laser arrangement and perform a wave front correction by deforming the mirror surface. Compare “Principle of Adaptive Optics”, Robert K. Tyson, Academic Press, 1991. The control bandwidth that is achievable in this manner is, however, limited by the mechanical characteristics of the adaptive mirrors.
Based on the above, one object of the invention is to provide an improved fiber laser arrangements of the type described above. More particularly, an object of the invention is to improve the beaming quality and eliminate the turbulence interferences occurring during the practical use of such fiber laser arrangements.
These and other advantages are achieved by fiber laser arrangement according to the invention, in which solid-state fiber lasers, particularly of laser-diode-pumped neodymium, herbium, or erbium glass fiber lasers, are fed from a fiber laser oscillator operated specifically in the longitudinal made. A fiber splitter arranged at the output of the fiber laser oscillator distributes the pumping energy generated by the fiber laser oscillator into a given number of optical fiber branches within the arrangement. Optical phase shifter elements which are assigned to the optical fibers, shifter elements, together with phase amplifiers in each of these branches, form the latter. This laser fiber arrangement is operated by an electronic control device which shifts the optical phases in the individual branches of the fiber laser arrangement in a manner that compensates for atmospheric turbulence effects on the propagation path of the laser radiation to the target. Thus, an optimal focusing of the entire laser radiation onto a remotely predefined target or a target point is achieved.
To obtain the control signals, the light scattered back from the target point at the propagation path to the transmitter (thus the fiber laser arrangement) is guided by a beam splitter to a wave front sensor, which measures the phase front of the light. From the measuring signal, the wave front phase-conjugated thereto is computed in an electronic control unit. Subsequently, the input signals are derived therefrom for controlling the optical phase shifter elements in the fiber laser arrangement, a fiber laser array.
A portion of the fiber laser radiation emitted by the fiber laser arrangement in the output beam is guided via a second beam splitter to a detector arrangement, and is coherently mixed there with the beam of a local oscillator laser. From the mixed signals of the individual detectors, the respective phase differences in the individual fiber laser branches are determined and are used in the electronic control system for the intended phase shifting. In this manner, an optimal phase coupling of the individual fiber laser branches of the arrangement is achieved for the desired intensity of the emitted laser radiation in the target point.
To obtain effective control signals, the target point, according to the invention, can also be exposed to the radiation of a separate illumination laser.
The phase-coupled fiber laser array according to the invention, consisting of a number of fiber laser amplifiers and fed by a common master oscillator, avoids the use of mirrors for compensating atmospheric turbulences. It also improves the beaming quality by targeted focusing of the laser radiation on a small spot size and, because of the division of the fiber laser array into individual fiber laser amplifier branches of any quantity, the total output of laser radiation that is emitted can be increased in a scalable manner, without any reduction of the focusing quality of the radiation in the target point because of turbulence interferences.
Possible applications of the invention are the optical application called “free air” over very long distances, optical countermeasures against sensors, as well as the use of lasers against flying targets.
The invention is described below by reference to the operating concept of an embodiment of a phase-coupled fiber laser arrangement with compensation of occurring turbulence interferences illustrated in the drawing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.